The present invention is involved in a preparation method of low-density color TPU foam by using compressed fluid as physical blowing agent. The obtained color TPU foam exhibits uniform pigment dispersion within polymer matrix, which is achieved by shortening the heating time of the as impregnated color TPU resin under low-temperature and high-pressure fluid, with the aim to reduce the interface migration of organic pigment molecules. The prepared color TPU foam has smooth surface, which is achieved by a gas compression process by reducing the pressure difference between inside and outside of cell structures, with the aim to prevent the non-uniform material shrinkage. The present invention is involved in the sintering of TPU bead foam and the slicing and compression molding of TPU foamed sheet, and the obtained foam material can be used in footwear applications.
Ethylene-vinyl acetate copolymer (EVA) loam is lightweight, flexible, and thus is used as insole, midsole, and outsole of sporting shoes. Color EVA foam endows sporting shoes with rich color configuration and fashion design ability. The preparation method of color EVA foam has been disclosed. One method is that mixing pigment with EVA particles, additives, fillers, chemical blowing agent, crosslinking agents together at low temperature, extruding them into small particle, foaming them within mold by heating, and thereby producing color EVA foam shoes. Another method is that putting the pigment into the EVA foaming system, and the foamed color EVA foam is obtained by injection foaming method. This method is well used for producing EVA foam sandals.
The used EVA resin for producing EVA foam is thermoplastic, which can be prepared into a desired shape by melt processing. However, the chemical crosslinking is required during EVA foaming, and the formed crosslinking chain structure can not be melt processed anymore. In addition, the used chemical blowing agent during EVA foaming usually produces toxic gases, thus the production process of EVA foam is non-environmental friendly; the residual chemical blowing agent leads to the properties instability of EVA foam.
Thermoplastic polyurethane (TPU) resin has the advantages of high wear resistance, high elasticity, fatigue resistance, chemical corrosion and so on, and is widely used in shoes. Color TPU foam is expected to replace color EVA foam to use in the high-performance shoes field.
The prior art can not prepare TPU foam with a rich and bright colors, with an uniform color distribution within and outside of material, with a low density of 0.05-0.5 g/cm3, with a uniform cell structure with the cell size less than 100 μm, and with the smooth surface without any wrinkles.
The method for preparing TPU foam by using a chemical blowing agent is known. The addition of pigments endows TPU foam with various color, but there are similar problems with the EVA foam, such as the requirement of chemical crosslinking and the presence of non-environmental friendly issues. In addition, the TPU foam prepared by using chemical blowing agent has others disadvantages such as rough cell structure, high density, and low expansion ratio.
In order to eliminate these mentioned drawbacks, Patent W02000/44821 A, W02004/108811A discloses the preparation method of TPU foam by using the expandable microspheres. During the processing, TPU resin in the form of powder or particle is mixed with expandable microspheres, and then carries on compression molding foaming to produce TPU foam. The microspheres are kinds of adhesion agent, or wax, or thermoplastic pellet wrapped with low boiling point solvent such as pentane inside. During the processing, the low boiling point solvent acts as the blowing agent to induce the formation of cell structure. The size of expandable microsphere is usually in the range of 20-40 μm, and they are difficult to well disperse into TPU matrix during the processing, and non-uniform cell structure is usually generated in TPU foam especially at high microsphere loading. For TPU foam with density of 0.3-1.0 g/cm3, the visible sag can be observed. Meanwhile, the as-prepared TPU foam exhibits poor wear resistance. The addition of pigments induces the formation of color TPU foam, but the prepared color TPU foam exhibits the similar problems, such as too large cell structure, coarse material surface, poor abrasion resistance property. Moreover, the prepared color TPU foam by this method does not possess the bright color.
Patent W020051066250A, W02007/082838A, W02010/136398A and CN103642200A described the preparation method of TPU foam by using autoclave and the organic solvent or inert as was used as physical blowing agent. The as-prepared TPU foam has small cell size, uniform cell structure distribution, low density up to 0.1-0.5 g/cm3, which solves the technical problems associated with the microsphere foaming method. However, the autoclave foaming method involves a cooking process of TPU granules within hot water with temperature of 100-150° C. and time of 0.5-10 h. TPU resin especially for the polyester-typed TPU resin is susceptible to degrade after long time water boiling, which affect the properties and sample appearance of TPU foam. More importantly, the organic molecular pigments are usually selected for TPU dyeing, and the organic molecular is prone to migrate under foaming agent plasticization long time hot water boiling, leading to the occurrence of color fading.
Patent W02007/082838A discloses a preparation meth of TPU foam by continuous extrusion foaming and underwater pelletization system. In this method, the saturated aliphatic hydrocarbons are the physical blowing agents and thus have the environmental problems and safety issues. The organic pigment is one kind of plasticizer, its addition usually decreases melt viscosity and melt strength, and thus reduce the foaming ability of TPU. Moreover, high open cell content will be generated during TPU extrusion foaming, leading to the occurrence of high shrinkage of TPU bead foam during sintering.
High pressure fluid such as high pressure CO2 is a clean, cheap and environmentally friendly physical blowing agent. At different conditions, high pressure CO2 fluid can be a gas fluid, a liquid fluid, and a supercritical fluid. The supercritical state of CO2 can be achieved under relatively mild condition, such as pressure of 7.31 MPa and temperature of 31.0° C.
Patent W02002/4628A describes a preparation method of high-density TPU foam using high-pressure fluid or supercritical fluid as blowing agent. The as-prepared TPU foam by this method has a hardness no less than 90 and density of 0.5-1.0 g/cm3, and it can be used as polishing pad of semiconductor wafer, but not can be used as footwear foam.
Patent CN102229709A describes a preparation method of TPU foam by using supercritical CO2 as a blowing agent. In this method, the used pressure is about 5-25 MPa, temperature of 100-250° C., and the treatment time of 1-60 min. Colored TPU foam can be prepared by using this method in theory, but the organic pigments is easy to immigrate under high temperature and pressure, leading to the occurrence of color fading. Furthermore, the applied high pressure in this method results in high equipment costs and safety issue. Moreover, this technique does not use a dispersion medium, TPU particles under high temperature and high pressure is easily deformed, the appearance becomes sticky, therefore, this technique is not suitable for the preparation of smooth, spherical or ellipsoidal shaped TPU foams.
International papers (Journal of Applied Polymer Sciences, 2010, 116, 1994; Journal of Applied Polymer Science, 2013, 128, 2245.) reported the preparation method of thermoplastic elastomer by using high pressure CO2 as blowing agent. In this method, thermoplastic elastomer was soaked by high pressure CO2 under low temperature, and the soaked materials were foamed by heating within water batch or steam. Thermoplastic elastomer possess high free volume and soft matrix, high CO2 diffusion rate leads to the occurrence of serious volume shrinkage of the foamed material up to 40-50%. The phenomenon is not only increasing the foam density but also inducing the formation surface folding, and thus affects its application.